i am looking for a "combat grave", sorry i do not know how to explain or name it in another way!

It is a M16 gun standing on top, rear in the ground and on top there is a helmet on it, has anybody seen something like that for arma or in it? I know there is a grave cross with a helmet on it BUT that is not what i am looking for ...

I bet it has been made for OFP (just guessing), but for Arma I am 95% sure its not available.
Maybe see if you can find it for OFP and see if you can convert it (ask permission).
Or........make one from scratch.

If ColonelSandersLite is right, and you own a copy of Operation Flashpoint: Cold war crisis. Then you already have the addon matey ;) Its just a matter of searching through the addons in the OFP directory.